Method for coating semiconductor device using droplet deposition

ABSTRACT

Methods and systems for coating of semiconductor devices using droplets of wavelength conversion or phosphor particles in a liquid medium. A plurality of nozzles delivers a controlled amount of the matrix material to the surface of the semiconductor device, with each of said nozzles having an opening for the matrix material to pass. The opening has a diameter wherein the diameter of the phosphor particles is less than or approximately equal to one half the diameter of the opening. The phosphor particles are also substantially spherical or rounded. The nozzles are typically arranged on a print head that utilizes jet printing techniques to cover the semiconductor device with a layer of the matrix material. The methods and systems are particularly applicable to covering LEDs with a layer of phosphor materials.

RELATED APPLICATION DATA

This application is a divisional of and claims the benefit of U.S.patent application Ser. No. 11/328,887 filed on Jan. 9, 2006 now U.S.Pat. No. 7,569,406.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to coating of semiconductor devices and to afabrication method for phosphor converted light emitting diodes (LEDs)for white and colored light generation. Specifically, the inventionrelates to methods for coating LEDs with a layer of phosphor particlesand LED package having LEDs coated using the methods.

2. Description of the Related Art

LEDs convert electric energy to light and they generally comprise anactive layer of semiconductor material sandwiched between two oppositelydoped layers. When a bias is applied across the doped layers, holes andelectrons are injected into the active layer where they recombine togenerate light that is emitted omnidirectionally from the active layerand from all surfaces of the LED. Recent advances in LEDs (such as GroupIII nitride based LEDs) have resulted in highly efficient light sourcesthat surpass the efficiency of incandescent of halogen light sources,providing light with equal or greater brightness in relation to inputpower.

Conventional LEDs do not generate white light from their active layers.One way to produce white light from conventional LEDs is to combinedifferent wavelengths of light from different LEDs. For example, whitelight can be produced by combining the light from red, green and blueemitting LEDs, or combining the light from blue and yellow LEDs. Thisapproach, however, requires the use of multiple LEDs to produce a singlecolor of light, increasing the overall cost and complexity. Thedifferent colors of light may also be generated from different types ofLEDs fabricated from different material systems. Combining different LEDtypes to form a white lamp can require costly fabrication techniques andcan require complex control circuitry since each device may havedifferent electrical requirements and may behave differently undervaried operating conditions (e.g. with temperature, current or time).

Light from a blue emitting LED has been converted to white light bysurrounding the LED with a yellow phosphor, polymer or dye, with atypical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG).[See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; Seealso U.S. Pat. No. 5,959,316 to Hayden, “Multiple Encapsulation ofPhosphor-LED Devices”]. The surrounding phosphor material “downconverts”the wavelength of some of the LED's blue light, changing its color toyellow. Some of the blue light passes through the phosphor without beingchanged while a substantial portion of the light is downconverted toyellow. The LED emits both blue and yellow light, which combine toprovide white light.

One method for combining an LED with a phosphor layer utilizesvolumetric dispense methods (e.g. dispensing using a syringe) forinjecting a large enough volume of phosphor containing encapsulatingresin or epoxy over the LED to cover the LED. For these methods,however, it can be difficult to control the phosphor layer's geometryand thickness. As a result, light emitting from the LED at differentangles can pass through different amounts of conversion material, whichcan result in an LED with non-uniform color temperature as a function ofviewing angle. Because the geometry and thickness is hard to control, itcan also be difficult to consistently reproduce LEDs with the same orsimilar emission characteristics.

Another conventional method for coating an LED is by stencil printing,which is described in European Patent Application EP 1198016 A2 toLowery. Multiple light emitting semiconductor devices are arranged on asubstrate with a desired distance between adjacent LEDs. The stencil isprovided having openings that align with the LEDs, with the holes beingslightly larger than the LEDs and the stencil being thicker than theLEDs. A stencil is positioned on the substrate with each of the LEDslocated within a respective opening in the stencil. A composition isthen deposited in the stencil openings, covering the LEDs, with atypical composition being a phosphor in a silicone polymer that can becured by heat or light. After the holes are filled, the stencil isremoved from the substrate and the stenciling composition is cured to asolid state.

Like the syringe method above, stencil method also presents difficultiesin controlling the geometry and layer thickness of the phosphorcontaining polymer. The stenciling composition may not fully fill thestencil opening such that the resulting layer is not uniform. Thephosphor containing composition can also stick to the stencil openingwhich reduces the amount of composition remaining on the LED. Theseproblems can result in LEDs having non-uniform color temperature andLEDs that are difficult to consistently reproduce with the same orsimilar emission characteristics.

Another conventional method for coating LEDs with a phosphor utilizeselectrophoretic deposition (EPD). The conversion material particles aresuspended in an electrolyte based solution. A plurality of LEDs areimmersed in the electrolyte solution. One electrode from a power sourceis coupled to the LEDs, and the other electrode is arranged in theelectrolyte solution. The bias from the power source is applied acrossthe electrodes, which causes current to pass through the solution to theLEDs. This creates an electric field that causes the conversion materialto be drawn to the LEDs, covering the LEDs with the conversion material.

After the LEDs are covered by the conversion material, they are oftenremoved from the electrolyte solution so that the LEDs and theirconversion material can be covered by a protective resin. This adds anadditional step to the process and the conversion material (phosphorparticles) can be disturbed prior to the application of the epoxy.During the deposition process, the electric field in the electrolytesolution can also vary such that different concentrations of conversionmaterial can be deposited across the LEDs. The conversion particles canalso settle in the solution which can also result in differentconversion material concentrations across the LEDs. The electrolytesolution can be stirred to prevent settling, but this presents thedanger of disturbing the particles already on the LEDs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, this invention relates to coating ofsemiconductor devices using droplets of wavelength conversion orphosphor particles in a liquid medium. One embodiment of a systemaccording to the present invention for depositing a layer of phosphormaterial comprises a semiconductor device, and a reservoir for holding amatrix material comprising phosphor particles mixed in a liquid medium.A plurality of nozzles is included each of which delivers a controlledamount of the matrix material to the surface of the semiconductordevice, and each of said nozzles having an opening for the matrixmaterial to pass. The opening has a diameter wherein the diameter of thephosphor particles is less than or approximately equal to one half thediameter of the opening. An apparatus imparts movement between thesemiconductor device and the nozzles to allow the delivery of the matrixmaterial over at least a portion of the surface of said semiconductordevice.

Another embodiment of a system according to the present invention fordepositing a layer of phosphor material comprises a semiconductordevice, and a reservoir for holding a matrix material having phosphorparticles. A print head is included over the semiconductor device andcomprising a plurality of nozzles, said matrix material provided to eachof said nozzles. Each of said nozzles comprises a nozzle opening and amechanism for causing a droplet of matrix material to be forced out ofthe nozzle through the opening and onto the semiconductor device. Thenozzle has an opening having a diameter, with the diameter of thewavelength conversion particles being one to two orders of magnitudesmaller than the diameter of the nozzle opening, and the wavelengthconversion particles being substantially rounded.

One embodiment of a method according to the present invention fordepositing a layer of phosphor material on a semiconductor devicecomprises providing a semiconductor device and providing a print headwith a plurality of nozzles. Each of the nozzles is capable ofdepositing a droplet of matrix material on the semiconductor device. Thesaid matrix material comprising a phosphor in a liquid medium. A layerof matrix material droplets is deposited from the nozzles on thesemiconductor device with a uniform pitch between adjacent droplets, andthe droplets are allowed to spread and flow together under gravity orsurface tension to form a layer of matrix material on the semiconductordevice. The liquid medium is allowed to evaporate leaving a layer ofessentially only phosphor particles on the semiconductor device.

Another method according to the present invention for depositing a layerof phosphor material on an LED comprises providing an LED, and providinga reservoir of matrix material comprising substantially sphericalwavelength conversion particles in a liquid medium. The matrix materialis formed into droplets, and the droplets are deposited on the LED withthe droplets forming a layer of phosphor particles on the LED.

One embodiment of a light emitter package according to the presentinvention comprises a solid state emitter having a plurality ofsemiconductor layers, the emitter emitting light in response to anelectrical bias. A layer of phosphor material covering at least aportion of the emitter, the layer comprising substantially roundedconversion particles deposited on the emitter through a print headnozzle. The layer of conversion material down-converts the wavelength ofat least some of said emitter light.

Another embodiment of a light emitter package according to the presentinvention comprises a solid state emitter having a plurality ofsemiconductor layers, the emitter emitting light in response to anelectrical bias. A layer of phosphor material covers at least a portionof the emitter, the layer comprising conversion particles deposited onthe emitter through a print head nozzle. The nozzle having an openingwherein the diameter of the conversion particles is less than orapproximately equal to half the diameter of the opening, said. The layerof conversion material absorbs and down-converts at least some of theemitter light.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of a phosphor deposition system according tothe present invention;

FIG. 2 shows one embodiment of a thermal bubble print head nozzle thatcan be used according to the present invention for depositing phosphors;

FIG. 3 shows one embodiment of a piezo crystal print head nozzle thatcan be used according to the present invention for depositing phosphors;

FIG. 4 is photo showing conventional phosphors;

FIG. 5 is a photo showing phosphors that can be used in the ink jetprinting systems according to the present invention;

FIG. 6 is a schematic illustrating the preferred size of phosphorparticles and nozzle opening in phosphor deposition systems according tothe present invention;

FIG. 7 is a sectional view of an LED with a layer of phosphor depositedaccording to the present invention in a volatile medium;

FIG. 8 is a sectional view of an LED with a layer of phosphor depositedaccording to the present invention in a non-volatile medium;

FIG. 9 is a sectional view of an LED with a layer of phosphor depositedaccording to the present invention in a curable medium;

FIG. 10 is a perspective view of an LED with phosphor/medium dropletsdeposited according to the present invention;

FIG. 11 is diagram showing phosphor/medium droplets deposited by a printhead according to the present invention; and

FIG. 12 shows an LED after coating with a phosphor according the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and apparatus for conformalcoating a semiconductor device with a matrix material comprised ofwavelength conversion particles in a liquid medium, with the method andapparatus using jet printing technology. In particular, the wavelengthconversion particles can comprise phosphor particles and the methods andapparatus can provide for conformal coating of a semiconductor devicewith the phosphor particles without the need for dispensing a volume ofphosphor containing encapsulant, or an electric field driven depositionprocess.

The present invention is described herein with reference to certainembodiments but it is understood that the invention can be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. The present invention is described hereinwith reference to a “droplet” that can be delivered to a semiconductordevice, and it is understood that this term includes any controlledamount of delivered to the surface of the semiconductor device,including a droplet, micro-droplet, stream, mist, etc.

The invention is particularly applicable to fabrication of white LEDemitters that generate white light by down converting blue or UV lightwith suitable phosphor materials. The present invention provides whiteLED emitters where the uniform conformal layer of phosphor particles isdeposited directly on the surface of the LED chip to provide a whitelight source with superior emission characteristics including angularuniform color of white emission (far field), spatially uniform color ofwhite emission from the LED source (near field), and a white source withmaximum luminance, i.e. emission is generated from a minimal sourcearea.

FIG. 1 shows one embodiment of a system 10 according to the presentinvention for depositing a layer of phosphor on a device, and inparticular an LED. The system 10 comprises a conventional ink jetprinter 12, which can be any number of printers with a suitable printerbeing model number DMP 2800 provided by Dimatix Incorporated. Ink Jetprinters are generally known in the art and are only briefly describedherein. Ink jet printing is a non impact printing method wherein ink isemitted from nozzles in a print head as the print head passes over themedia (e.g. paper or an LED as described herein). Liquid is held in areservoir (typically in the print head) and is delivered to and sprayedfrom the nozzles onto the media as the print head scans the media inhorizontal strips. At the same time the media is moved forward in stepsunder the print head. A strip of the liquid is deposited on the media ina strip and the media is moved for printing of the next strip. In someembodiments, the print head can deposit multiple strips of liquid ineach pass by having multiple print heads. In other embodiments the printhead can be angled to deposit on angled or vertical surfaces, ormultiple print heads can be provided one or more of which can be angled.

The printer 12 typically operates in communication with and under thecontrol of a computer 14, which is typically a conventional personalcomputer (PC), although other computers can also be used. The controlsignals and pattern to be printed are transferred from the PC to theprinter over a standard communications bus 16. The pattern for printingcan be generated on the PC 14 using software such graphics softwareresident on the PC 14. Alternatively the pattern can be loaded on the PC14 from a peripheral device such as compact disk 18, a scanner 20, or aserver 22. The server 22 can provide information from a computer networkor from over the Internet. When the appropriate data is loaded in theprinter 12 from the PC 14, and the LED (or other device) 24 is properlyloaded in the printer 12, the print head 26 can deposit a layer ofphosphor material on the LED 24.

Jet print heads typically contain a series of nozzles each of which isarranged to deposit droplets of liquid on the LED 24 in the printer 12.Depending on the manufacturer and model of the printer, liquid (ink)cartridges are used as the supply of liquid to be deposited on themedia, and in some models the cartridges include the print head itself.Different types of printers form droplets using different types ofnozzles. FIG. 2 shows one known nozzle 30 commonly referred to as bubblejet or thermal bubble in steps of its droplet formation. This technologyuses tiny resistors 32 located in the liquid holder 34 of each nozzle30. A signal is applied to the resistors 32 to create heat thatvaporizes the liquid (ink) in the holder 34 to form a bubble 36. As thebubble 36 expands, some of the ink is pushed out of the nozzle opening37 in the form of a droplet 38. When the bubble 36 pops (collapses) avacuum is created that pulls more ink into the nozzle reservoir 34.

FIG. 3 shows another common nozzle 40 that uses a piezo crystal 42,located such that its movement under a bias causes formation of a liquiddroplet 46. An electrical signal is applied to the piezo crystal 42causing it to vibrate. When the crystal 42 vibrates inward, a controlledamount of liquid is forced out of the nozzle 40, through the nozzleopening 44 and in the form of a droplet 46. When the piezo crystal 42vibrates out, liquid is pulled into the nozzle 40 from the reservoir 48to replace the liquid forced out. Typical print heads having thermalbubble and piezo crystal nozzles can have 300 to 600 tiny nozzles, allof which can be fired simultaneously, or sequentially as needed. Thetypical diameter for each nozzle opening is in the range of 30-50microns.

FIG. 4 shows typical phosphor particles 50 (such as YAG:Ce garnets) thatrange in size from 3 to 30 microns or larger. In the past, jet printingsystems using phosphors of this size and shape have been hampered bypoor equipment and process reliability. The size of these particles canclog the print head nozzles, and in particular those particles having adiameter that is greater than one half the diameter of the nozzleopening. The danger of clogging increases as the particle diametersapproach and exceed the size of the nozzle opening. The sharp edges ofthese particles also can be problematic. For particles being smallenough to pass through a nozzle, the sharp edges act as an abrasive atthe nozzle opening and can damage the nozzle. Using this particle thenozzle can fail in a relatively short time.

FIG. 5 shows phosphor particles 52 that can more reliably be used inphosphor jet printing deposition processes according to the presentinvention. The phosphors are significantly smaller and lack the sharpedges. These phosphors can range in size from approximately ½ to 10microns, with a preferred range of sizes being 1-6 microns. Thesephosphors have a substantially rounded or spherical shape without sharpedges. Phosphor particles of this type can be fabricated using differentmethods, with one suitable method being spray pyrolysis. Pyrolysisgenerally involves atomizing an aqueous solution of precursors in astream, and the stream consisting of droplets suspended in a carrier gasis passed through a tubular furnace. In the furnace, the precursorreacts in the solid phase, forming the final product powder of phosphorparticles in the desired shape and size. Using these particle can resultin more precise and repeatable fabrication of white emitting LEDs.

FIG. 6 shows a print head nozzle 60 according to the present inventionthat has a nozzle diameter (d₁) 62, and a phosphor particle 64 having adiameter (d₂) 66. For phosphor particles to be reliably used indepositing phosphors according to the present invention, the particlediameter 66 and nozzle diameter 62 should have a size relationship thatallows the phosphor particles to easily pass through the nozzle withoutclogging. In one embodiment according to the present invention, thephosphor particle diameter 66 should be less than or approximately equalto one half the nozzle diameter 62. In another embodiment, the particlediameter 66 should be one to two orders of magnitude smaller than thenozzle diameter 62. The particles should also lack sharp edges and besubstantially spherical and round as described above so that the nozzleis not damaged by sharp edges as the phosphor passes through. Althoughthe particles do not need to be spherical (as shown in FIG. 5), is itpreferable that the particles be substantially spherical, or as close tospherical as possible to avoid damage to the nozzle. It can beappreciated from this size relationship between the nozzle diameter andparticle diameter that the smaller phosphor particle diameter, thesmaller the nozzle diameter that can be used. As nozzles are developedwith larger diameters, phosphors of a larger diameter can be used.

The phosphor particles can be one or more fluorescent or phosphorescentmaterials such as a phosphor, fluorescent dye or photoluminescentsemiconductor. The following lists of some of the phosphor particlesthat can be used in the methods and apparatus according to the presentinvention, grouped by the re-emitted color following excitation:

Red

-   -   Y₂O₂S:Eu³⁺,Bi³⁺    -   YVO4:Eu³⁺,Bi³⁺    -   SrS:Eu²⁺    -   SrY₂S₄:Eu²⁺    -   CaLa₂S₄:Ce³⁺    -   (Ca, Sr)S:Eu²⁺    -   Y₂O₃:Eu³⁺,Bi³⁺    -   Lu₂O₃:Eu³⁺    -   (Sr_(2-x)La_(x))(Ce_(1-x)Eu_(x))O₄    -   Sr₂Ce_(1-x)Eu_(x)O₄    -   Sr_(2-x)Eu_(x)CeO₄    -   Sr₂CeO₄    -   SrTiO₃:Pr³⁺,Ga³⁺

Orange

-   -   SrSiO₃:Eu,Bi

Yellow/Green

-   -   YBO₃:Ce³⁺,Tb³⁺    -   BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺    -   (Sr,Ca,Ba)(Al,Ga)₂S₄:Eu²⁺    -   ZnS:Cu⁺,Al³⁺    -   LaPO₄:Ce,Tb    -   Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺,Mn²⁺    -   ((Gd,Y,Lu,Se,La,Sm)₃(Al,Ga,In)₅O₁₂:Ce³⁺    -   ((Gd,Y)_(1-x)Sm_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce³⁺    -   (Y_(1-p-q-r)Gd_(p)Ce_(q)Sm_(r))₃(Al_(1-y)Ga_(y))₅O₁₂    -   Y₃(Al_(1-s)Ga_(s))₅O₁₂:Ce³⁺    -   (Y,Ga,La)₃Al₅O₁₂:Ce³⁺    -   Gd₃In₅O₁₂:Ce³⁺    -   (Gd,Y)₃Al₅O₁₂:Ce³⁺,Pr³⁺    -   Ba₂(Mg,Zn)Si₂O₇:Eu²⁺    -   (Y,Ca,Sr)₃(Al,Ga,Si)₅(O,S)₁₂    -   Gd_(0.46)Sr_(0.31)Al_(1.23)O_(x)F_(1.38):Eu²⁺ _(0.06)    -   (Ba_(1-x-y)Sr_(x)Ca_(y))SiO₄:Eu    -   Ba₂SiO₄:Eu²⁺

Blue

-   -   ZnS:Ag,Al

Combined Yellow/Red

-   -   Y₃Al₅O₁₂:Ce³⁺,Pr³⁺

White

-   -   SrS:Eu²⁺,Ce³⁺,K⁺

From the list above, the following phosphors are most suitable for usein deposition on LEDs having excitation in the blue and/or UV emissionspectrum, by providing a desirable peak emission, having efficient lightconversion, and by having acceptable Stokes shift:

Red

-   -   Lu₂O₃:Eu³⁺    -   (Sr_(2-x)La_(x))(Ce_(1-x)Eu_(x))O₄    -   Sr₂Ce_(1-x)EU_(x)O₄    -   Sr_(2-x)Eu_(x)CeO₄    -   SrTiO₃:Pr³⁺,Ga³⁺

Yellow/Green

-   -   (Sr,Ca,Ba)(Al,Ga)₂S₄:Eu²⁺    -   Ba₂(Mg,Zn)Si₂O₇:Eu²⁺    -   Gd_(0.46)Sr_(0.31)Al_(1.23)O_(x)F_(1.38):Eu²⁺ _(0.06)    -   (Ba_(1-x-y)Sr_(x)Ca_(y))SiO₄:Eu    -   Ba₂SiO₄:Eu²⁺

As mentioned above, the uniform coating of LEDs according to the presentinvention is provided by jet printing LEDs with a conformal coat ofphosphor material with the phosphor being mixed with and delivered in aliquid medium. The phosphor particles can be mixed in many differentmediums and held in a print head reservoir for deposition according tothe present invention. In one embodiment, the phosphor particles can besuspended in a volatile organic or inorganic medium that dries shortlyafter the phosphor/medium mixture is deposited. FIG. 7 shows oneembodiment of an emitter 70 comprising an LED 71 covered with a layer ofphosphor particles 72 that were deposited in a volatile medium. Afterdeposition, the volatile medium evaporates, leaving only the layer ofphosphor particles 72. Air gaps 74 can form between adjacent particles,and because of the difference in the index of refraction for air and thesurrounding semiconductor and phosphor materials, the air gaps canreduce light extraction efficiency. To fix the phosphor particles 72 inplace they can then be covered by a layer of curable material 76, suchas an epoxy, silicone or glass. When the particles are covered, it isdesirable for the curable material to fill the air gaps 74 to reduce thenegative impact on light extraction. Many different volatile mediums canbe used, including but not limited to alcohol, water, isopropynol,hydrocarbon solvents, hexane, methanol, methyl ethyl katone, ethyleneglycol, and combinations thereof.

FIG. 8 shows another embodiment of an emitter 80 also comprising an LED81 covered with a layer of phosphor particles 82 delivered in a phosphorparticle non-volatile medium mixture. The non-volatile medium can beorganic or inorganic and after delivery of the mixture, the particles 82remain suspended in a non-volatile medium to form a phosphor particlelayer 83. Many different materials can be used for the non-volatilemedium including but not limited to silicon oils and hydrocarbon oils.When the non-volatile medium and phosphor is deposited the phosphorparticles remain immobilized and fixed in place by the medium. Thedevice can then be encapsulated by an epoxy, silicone or glass layer 84that covers the phosphor particle layer 82. One advantage of thisembodiment is that the likelihood of air gaps forming in the phosphorlayer 82 is greatly reduced because the non-volatile medium tends tofill all the gaps.

FIG. 9 shows another embodiment of an emitter 90 comprising an LED 91having phosphor particles 92 delivered in a phosphor particle curableorganic or inorganic medium mixture. The curable organic or inorganicmedium can comprise any number of curable materials such as silicone orepoxy. The resulting particle curable medium layer 94 can be depositedon the device to form a single epoxy phosphor layer that can then becured. The curing of the layer 94 fixes the phosphor particles in place,while at the same time providing a hardened protection layer. The devicecan then be over coated with an additional encapsulating layer 96 foradditional protection. Further, curable or non-curable non-volatilematerials can be delivered also in combination with volatile mediumswhich can act as solvents for them.

Each of the emitters 70, 80 and 90 can also have scattering particles 98(shown in FIG. 9) to help scatter emitting light. The particles layersare shown in FIG. 9 in the encapsulating layer 96, but also can bedeposited with the phosphor particle layers, or in both. Many differenttypes of scattering particles can be used, with the preferred scatteringparticles being made of barium sulfate or titanium dioxide.

For each of the phosphor medium mixtures described above and showndelivered on the emitters in FIGS. 7-9, the mixture should have aviscosity that allows it to be efficiently deposited by the a jet printhead such as those described above and shown in FIGS. 2 and 3. If theviscosity is too low its surface tension can be low enough that themixture can run out of the print head through the nozzle openings. Ifthe viscosity is too high, the force from thermal bubble or piezocrystal, as the case may be, may be insufficient to force a drop ofmixture through the nozzle. Many different ranges of viscosity can beused depending on the diameter of the nozzle opening and the force ofthe thermal bubble or piezo crystal. For conventional piezo print heads,a suitable range of viscosity is 8-20 centipoise (cp), although othermixture viscosities can also be used. If the viscosity is not within asuitable range, solvents can be added to lower the viscosity, or fillerscan be added to increase the viscosity. Examples of volatile solventsthat can be used to include water or methyl ethyl ketone.

The use of conventional sized phosphor particles in ink jet printingalso presented a problem with settling of the phosphor in the phosphormedium mixture. Settling can result in layers of different phosphorconcentration being deposited, which can reduce the repeatability of thephosphor deposition. Smaller particles according to the presentinvention and shown in FIG. 4 provide the additional advantage ofremaining suspended longer in the medium having a viscosity in the rangedescribed above. This allows for more repeatable deposition of thephosphor layer.

Another aspect of the present invention provides for the coating of asemiconductor chip (LED) surface with thin layer(s) of phosphorcontaining fluids by precisely targeting and depositing droplets of thefluids repeatedly at a frequency on predetermined spots on the surface.A short time is then provided for the fluids droplets to spread and flowinto adjacent droplets under gravity or surface tension to form a thinfilm on the surface. The chemical fluids may be made more viscous orsolidified to form a permanent coating by evaporating off the fluidsolvents of chemical changes initiated by heat or radiation.

FIG. 10 shows one embodiment of an emitter 100 according to the presentinvention partially covered by droplets 108 of a phosphor particleliquid medium mixture. The LED 100 comprises an LED 101 on a submount102, with a first bond wire 104 coupled to the submount 102 and a secondwire bond 106 coupled to the LED 101. A bias applied across the wirebonds 104, 106 is applied to the LED 101 to cause it to emit light. Thedroplets 108 can comprise different mixtures according to the presentinvention, with the preferred mixture being one of the mixturesdescribed above in the description of the emitters in FIGS. 7-9. Solidpowders, such as phosphor particles, that are designed to coat the chipto perform certain functions on the chip are preferably spherical shapesof micron or sub-micron sizes, although the powders can also be certainother irregular shapes.

After the droplets 108 are deposited on the LED 101, gravity and surfacetension causes the droplets to run together to form a layer of phosphorparticle medium mixture. Depending on the size of the droplets 108 andthe distance between adjacent droplets 108 (pitch), droplets will formlayers of different thickness. The LED 101 is shown with droplets 108having a pitch of 0.05 millimeters (mm) on the top surface, and dropletswith a pitch of 0.02 mm on another surface. These different pitches forthe droplets can result in mixture layers of different thickness.

The droplets 108 are preferably positioned and deposited on the LED 101using jet printing as described above. Jet printing technology, providesdroplets 108 of matrix material comprising phosphor particles in aliquid medium. The droplets 108 are deposited to form a coating of adesired thickness and the layer thickness can be positively controlledby the size of the nozzle, pitch between adjacent droplets, andviscosity of the droplets. FIG. 11 shows a jet print head 110 positionedover the emitter 111 that is similar to the emitter 100 in FIG. 10, andis shown as it would be arranged in one printing system according to thepresent invention. The droplets 112 can be deposited from the print head110 through nozzles such as those described above and the matrix can beany one of the phosphor particle medium mixtures described above such asphosphor particles in a volatile, non-volatile or curable medium.

FIG. 11 shows droplets being deposited on the top surface of an LED 112with a 0.05 mm pitch and a 0.03 pitch between droplets. The differentpitch can be controlled by having a print head with the appropriatedistance between adjacent nozzles. Alternatively, the movement of theprint head or LED under the print head can be controlled to give theappropriate droplet pitch. By using a layer of droplets with acontrolled distance between adjacent droplets, a layer of controlledthickness can be formed. Droplets can be deposited on the angledsurfaces 114 and vertical surfaces 116 of the LED 112 using angled printheads (not shown). Alternatively, larger droplets can be deposited onthe top surface of the LED 112 and as gravity and surface tension causethe droplets to form a layer, the droplet fluids can run down the angledand vertical surfaces 114, 116.

The droplets are preferably delivered in phosphor particle volatileliquid medium, with the medium evaporating shortly after the dropletsare deposited and run together. After the volatile medium evaporates theremaining phosphor particles typically form a phosphor layer that is afraction of a micron thick. If this is the desired thickness for thelayer, the LED can be encapsulated to fix the phosphors. To form thickerlayers, the print head can pass over and deposit additional layers tobuild up the desired thickness. The pitch, viscosity and print headspeed on provided by each pass of the print head can determine thethickness of each layer as the layers are built up.

FIG. 12 shows one embodiment of an emitter 120 also comprising an LED122 mounted to a submount 124. The LED 122 is coated by a layer 126 ofphosphor particles according to the present invention. Differentembodiments of the phosphor particles material can have differentcharacteristics. In one embodiment, phosphor particle layer 126 canabsorb some wavelengths of light emitted by the LED and re-emit otherwavelengths, thus resulting in different colors. In another example,barium sulfate or titanium dioxide (scattering) particles that aresuspended in a clear silicone or epoxy will disperse light rays emittedby the LED chip in all directions. In still another example, nanoparticles of certain chemicals when coated LED may actually modify thewavelengths emitted by the LED.

The invention provides uniform layer(s) of coating having few micronsthickness and can be formed precisely and efficiently on semiconductorchips that have been wire bonded, without breaking either the chip orthe wires. In one embodiment, a blue LED chip can be coated with thin,uniform layers of yellow phosphors that produce a more uniform whitelight and higher efficacy (light output/power output) than an LEDcovered using prior injection or EPD methods. Jet printing techniquescan be used to produce droplets for coating, which have an inherentlybetter material efficiency and productivity than conventional dispensingor electrophoretic deposition techniques, hence a lower cost. Theprocess of coating can be done at a high speed with print heads thattypically have hundreds of nozzles; and a printer may be equipped with aplurality of print heads.

Precise phosphor placement on shaped LED chips is possible with ink jetnozzle arrays, with some arranged at varying angles. Small dispensevolumes in the range of pico-liters allow for spatial variation of thephosphor deposition across the chip, if desired. Jet printing offers theflexibility in dispense capability to place conformal coating of variousphosphors either simultaneously or as a layer sequence on the LED chipto achieve emission at various color points.

Although the present invention has been described in detail withreference to certain preferred configurations thereof, other versionsare possible. The printer according to the present invention can be usedto coat many devices beyond LEDs and can deposit materials beyondphosphors. Therefore, the spirit and scope of the invention should notbe limited to the versions described above.

1. A system for depositing a layer of phosphor material, comprising: asemiconductor device; a reservoir for holding a matrix materialcomprising phosphor particles mixed in a liquid medium; a plurality ofnozzles each of which is arranged to deliver a controlled amount ofdroplets of said matrix material to the surface of said semiconductordevice, each of said nozzles having an opening for said matrix materialto pass through, said opening having a diameter wherein the diameter ofsaid phosphor particles is less than or approximately equal to one halfthe diameter of said opening; and an apparatus to impart movementbetween said semiconductor device and said nozzles to allow the deliveryof said matrix material over at least a portion of said surface of saidsemiconductor device.
 2. The system of claim 1, wherein saidsemiconductor device is an LED.
 3. The system of claim 1, wherein saidliquid medium comprises liquid from the group comprising volatileliquid, non-volatile liquid and curable liquid.
 4. The system of claim1, wherein said phosphor particles are substantially rounded.
 5. Thenozzle of claim 1, wherein said controlled amount of matrix materialfrom each of said nozzles comprises a droplet.
 6. A system fordepositing a layer of phosphor material, comprising: a semiconductordevice; a reservoir for holding a matrix material having wavelengthconversion particles; and a print head over said semiconductor deviceand comprising a plurality of nozzles, said matrix material provided toeach of said nozzles, wherein each of said nozzles comprises: a nozzleopening; and a mechanism for causing a droplet of said matrix materialto be forced out of said nozzle through said nozzle opening and ontosaid semiconductor device, said nozzle opening having a diameter, thediameter of said wavelength conversion particles being one to two ordersof magnitude smaller than said diameter of said nozzle opening and saidwavelength conversion particles being substantially rounded.
 7. Thesystem of claim 6, further comprising an apparatus to impart movementbetween said print head and said semiconductor device to allow saidnozzles to deposit a layer of said matrix material on said semiconductordevice.
 8. The system of claim 6, wherein said semiconductor devicecomprises a light emitting diode (LED).
 9. The system of claim 6,further comprising a liquid medium, said matrix material comprising amixture of said liquid medium and said wavelength conversion particles.10. The system of claim 9, wherein said liquid medium comprises avolatile, non-volatile or curable liquid.
 11. The system of claim 6,wherein said nozzles are arranged to deposit droplets of said matrixmaterial on the surface of said semiconductor device with a controlledpitch between adjacent droplets, said droplets running together to forma layer of said matrix material on said semiconductor device.
 12. Thesystem of claim 6, wherein said wavelength conversion particles comprisephosphor particles.